In the field of lubricating oil used in applications such as internal combustion engine and automatic transmission, in recent years, from such points of view as efficient use of resources, decrease in waste oil and cost reduction for users of lubricating oil, requirements have increased regarding lubricating oils to have longer drain intervals. Thus, in prior art lubricating oils, suppression of deterioration of lubricating oils has been devised by mixing of various additives. For instance, in the case of a lubricating oil for internal combustion engine, the combined use of a sulfur-containing compound, such as, zinc dithiophosphate (ZDTP) or molybdenum dithiocarbamate (MoDTC), and a reaction chain terminator, such as, a phenol series or an amine series antioxidant, is generally considered to be effective from the point of oxidation stability, such that ZDTP is recognized broadly in general as an essential additive to be mixed in large amounts to some extent. In particular, deterioration of lubricating oil being pronounced in high output engines, such as, gas engines and direct injection engines, with high combustion temperatures and higher NOx gas concentrations contaminating the lubricating oil, the combined use of ZDTP and phenol series and/or amine series antioxidants is considered to be essential.
However, as the price to pay for decomposing peroxides, a sulfur-containing compound such as ZDTP may per se oxidize or thermally decompose and become the cause of acidic substance generation, such as of sulfuric acid, therefore, in lubricating oils for internal combustion engine, or the like, where metallic cleanser and ashless dispersant are mixed in general, it becomes a cause that triggers a decrease in base number, which is an indicator for acid neutralization property, and a decrease in cleaning properties at high temperature. Consequently, as long as large amounts of sulfur-containing compounds such as ZDTP are mixed, rendering [the oil] to have additionally longer drain intervals is extremely difficult to achieve.
On the other hand, in general, decreasing the quantity of metallic cleanser mixed in the lubricating oil is considered to be necessary in order to prevent ash deposition onto pistons of internal combustion engines, catalysts, such as, three-way catalysts, oxidation catalysts and NOx storage reduction type catalysts, or emission gas purification devices, such as, DPF; however, simply decreasing the quantity of mixed metallic cleanser compromises acid neutralization properties and cleaning properties at high temperature.
Furthermore, in regard to the above-mentioned catalysts, in particular the NOx storage reduction type catalyst, in order to decrease poisoning due to sulfur, studies on reducing sulfur content in fuels (for instance, light oil with a sulfur content of 50 mass ppm or less, gasoline with a sulfur content of 10 mass ppm or less, and the like) are proceeding rapidly, and the effects thereof are anticipated; however, in an internal combustion engine using such low sulfur fuels, the influence on the catalysts exerted by the sulfur content in the lubricating oil become relatively important. Consequently, a further increase in the amounts of sulfur-containing compounds in the lubricating oil is not desirable, and decreasing the sulfur content in the lubricating oil is essential.
Thus, studies were made to solve the above-mentioned problematic points in conventional lubricating oils and achieve sufficiently longer drain intervals. For instance, disclosed are, in Patent Reference 1, a lubricating oil composition in which a specific phosphorus compound has been mixed, in Patent Reference 2, a lubricating oil composition in which the content of ZDTP has been decreased and at the same time salicylate and sulfonate and a phosphorus-containing anti-wear agent are used in combination, and, in Patent Reference 3, a lubricating oil composition in which a specific phosphorus compound and a sulfur-containing organic molybdenum complex are used in combination, respectively.    [Patent Reference 1] Japanese Patent Application Laid-open No. 2002-294271    [Patent Reference 2] Japanese Patent Application Laid-open No. 2003-277781    [Patent Reference 3] Japanese Patent Application Laid-open No. 2004-83891